In the known solutions of pulverized coal firing the pulverized coal ground to the required fineness in a pulverized coal preparation system and dried therein is fed to a pulverized coal burner where the pulverized coal is mixed with the combustion air. The mixture ignites in the combustion chamber and burns at a high temperature. A part of the released heat quantity is radiated at the cooled wall of the furnace chamber while the remaining part will be utilized at the other surfaces of the boiler (super-heater, feed water preheater, air heater).
This known mode of combustion and the plant serving for its realization are disadvantageous from several points of view. It is disadvantageous that in order to obtain practically complete combustion of the combustible part of the pulverized coal, the coal must be ground to the required fineness. Grinding involves a significant expenditure of energy and costs. Even despite this measure, one must reckon with ignition problems and with extinction of the fire when the quality of the coal is poorer than planned. A further disadvantage is that the still tolerable partial load (without auxiliary combustion with gas or oil) amounts to approximately 60 to 70% of the nominal load. It is also disadvantageous that due to the high temperature prevailing in the furnace chamber, the emission of environmentally harmful materials, particularly of NO.sub.x, has an extremely high value and the basic additive for use for binding the SO.sub.2 is not sufficiently effective so that a supplementary, expensive flue gas purification must be carried out.
In the per se also known solutions of fluidized bed combustion, the unground coal introduced e.g. via an air distributor assuring uniform distribution is combusted completely or partially with the combustion air flowing through the air distributor at a relatively low temperature of between 750.degree. and 950.degree. C.
In this case, the grinding process producing the pulverized coal is obviated. Thereby the costs of grinding are saved, the glowing fluidized bed of high mass assures an insensitivity to changes in the quality of the coal and at the same time the combustion temperature, being restricted to the temperature range of between 750.degree. and 950.degree. C., also restricts environmental pollution caused by NO.sub.x and SO.sub.2.
The known mode of using fluidized bed combustion nevertheless brings also certain disadvantages with it.
In a variant of fluidized bed combustion, a full combustion of the coal is realized in the fluidized bed and the overwhelming part of the released heat is abstracted by way of cooling surfaces immersed into the fluidized bed. In another version the coal is only partially combusted in the fluidized bed whereafter the gases leaving the fluidized bed are combusted with air passed over the fluidized bed. In this solution the cooling surfaces immersed into the fluidized bed are mostly dispensed with.
In both cases the flow velocity of the air supplied to maintain the fluidized state and also to promote the combustion process is relatively low (it is, in general, below a velocity of 4 m/s). Consequently it is necessary to overdimension the cross-section of the fluidized bed transversely of the direction of air flow in relation to the dimensions of the pulverized coal combustion systems wherein the velocity of the pulverized coal supply does not in general exceed the value of 15 m/s; in other words, for a unit surface area of the air distributor only a relatively small amount of heat can be released which again leads to a significant increase of dimensions.
Hybrid combustion systems combining these known systems have been developed at least partially to obviate the drawbacks and disadvantages of fluidized bed combustion and the conventional pulverized coal combustion. The known apparatuses for realizing this hybrid combustion nevertheless do not enable the frequently necessary strong output control to be carried out, and in addition the sensitivity to the quality of the coal is the same as before, both combustion systems being served by the same coal grinding and delivery system even when it contains separate units. A typical example of such a hybrid combustion system may be learned from Hungarian Patent Specification No. 185 694.
In this apparatus the relatively homogeneous, grit-like coal, pre-dried in the coal mill, separated in an air separator connected downstream of the coal mill and ground to a predetermined, e.g. 1-3 mm particle size, is not recycled to the coal mill for further comminution; instead, it is fed with the aid of a mechanical transport device to a fluidized bed formed at the bottom of the furnace chamber of the boiler, while the finely ground pulverized coal passing the air separator is fed from the air separator via a pulverized coal duct into a pulverized coal burner debouching into the combustion chamber. In this solution the coal mill and the air separator used are such as to permit the whole coal quantity required for both systems to pass therethrough.
A further disadvantage of this solution consists in that the portion of the coal combusted in the fluidized bed and the portion of the coal combusted in the pulverized coal burner are fixed and the operation of the two systems cannot be separated from each other.
The aim of the invention is the combination of the advantages of the known solutions with the simultaneous obviation of their drawbacks.